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Astrophysics A&A 447, 465–472 (2006) Astronomy DOI: 10.1051/0004-6361:20053819 & c ESO 2006 Astrophysics Large-scale magnetized outflows from the Virgo Cluster spiral NGC 4569 A galactic wind in a ram pressure wind K. T. Chy˙zy1, M. Soida1,D.J.Bomans2, B. Vollmer3,Ch.Balkowski4, R. Beck5, and M. Urbanik1 1 Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland e-mail: [email protected] 2 Astronomisches Institut, Ruhr-Universität-Bochum, 44780 Bochum, Germany 3 CDS, Observatoire astronomique de Strasbourg, UMR 7550, 11 rue de l’Université, 67000 Strasbourg, France 4 Observatoire de Paris, GEPI, CNRS UMR 8111, and Université Paris 7, 5 Place Jules Janssen, 92195 Meudon Cedex, France 5 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany Received 12 July 2005 / Accepted 3 October 2005 ABSTRACT Using the Effelsberg radio telescope at 4.85 GHz and 8.35 GHz we discovered large symmetric lobes of polarized radio emission around the strongly Hi deficient Virgo cluster spiral galaxy NGC 4569. These lobes extend up to 24 kpc from the galactic disk. Our observations were complemented by 1.4 GHz continuum emission from existing Hi observations. This is the first time that such huge radio continuum lobes are observed in a cluster spiral galaxy. The eastern lobe seems detached and has a flat spectrum typical of in-situ cosmic ray electron acceleration. The western lobe is diffuse and possesses vertical magnetic fields over its whole volume. The lobes are not powered by an AGN, but probably by a nuclear starburst that occurred ∼30 Myr ago, producing ≥105 supernovae. Since the radio lobes are symmetric, they resist ram pressure due to the galaxy’s rapid motion within the intracluster medium. Key words. galaxies: individual: NGC 4569, IC 3583 – galaxies: magnetic fields – radio continuum: galaxies 1. Introduction pressure stripping event (the inclination angle between the disk and the orbital plane is 35◦), where the maximum ram pressure NGC 4569 is a bright spiral galaxy (Sb) whose projected angu- occurred ∼300 Myr ago, are consistent with the Hi observations . ◦ = lar distance to the Virgo Cluster center (M 87) is only 1 7 (Vollmer et al. 2004). 1 0.5Mpc . Because of its brightness and large diameter (D25 = ff Tschöke et al. (2001) discovered a di use extraplanar re- 9.5 = 47 kpc), Stauffer et al. (1986) have questioned its α i gion of X-ray and H emission to the west of the galactic disk. cluster membership. NGC 4569 has about one tenth the H con- This was the first evidence of a direct connection between the tent of a field galaxy of the same morphological type and the hot X-ray gas and the Hα emission at scales of 10 kpc. This ion- same size (Giovanelli & Haynes 1983). It shows a strongly −1 i ized gas is flowing out from the disk at a velocity of 120 km s truncated H disk (Cayatte et al. 1990), most probably a sig- (Bomans et al. 2005). The most probable source of this outflow nature of strong stripping by the intracluster medium which is a central starburst (Barth & Shields 2000; Tschöke et al. pervades the Virgo cluster (Cayatte et al. 1994). Tschöke et al. 2001). Present-day AGN activity is ruled out due to the lack − (2001) did not find any soft X-ray emission (0.1 0.4 keV) from of a compact point source in the ASCA hard band (Tschöke the northern half of the disk, whereas it is pronounced in the et al. 2001), a missing radio continuum point source (Neff & southern disk (cf. Fig. 1). These findings are consistent with a Hutchings 1992; Hummel et al. 1987), and spectral synthesis ram pressure scenario where the galaxy is moving to the north- analysis (Barth & Shields 2000). Gabel & Bruhweiler (2002) α east through the intracluster medium. The H emission distri- dated the nuclear starburst in the inner ∼30 pc to 5−6Myr bution is, as is Hi, sharply truncated at 30% of the optical ra- i based on optical and UV HST spectra. In addition, Keel (1996) dius (Koopmann et al. 2001). In addition, an anomalous H and found a more extended (∼300 pc) population of A-type super- Hα arm is detected to the west of the galactic disk. Simulated ∼ i giants whose age is greater than 15 Myr. Thus the nucleus of H gas distributions and velocity fields of a more edge-on ram NGC 4569 contains at least two distinct young stellar popula- tions: a very young UV core (5−6 Myr) and a spatially more 1 We use a distance to the Virgo cluster of D = 17 Mpc. extended region dominated by A supergiants. Article published by EDP Sciences and available at http://www.edpsciences.org/aa or http://dx.doi.org/10.1051/0004-6361:20053819 466 K. T. Chy˙zy et al.: Large-scale magnetized outflows from the Virgo Cluster spiral NGC 4569 In this work we use radio polarimetry to study the gas from both horns, using the “software beam-switching” tech- outflows from NGC 4569. Total power emission and polarized nique (Morsi & Reich 1986). This was followed by restoration radio continuum emission represent very sensitive tracers of of total intensities (Emerson et al. 1979), map transformation flows of diffuse magnetized gas, often unnoticed in the Hi and to RA/Dec coordinates and spatial frequency-weighted combi- Hα lines or in X-rays (Beck et al. 1999; Soida et al. 2002). nation leading to the final Stokes I map. While the polarization allows one to study the magnetic field Because of azimuthal mounting of the radio telescope the Q in the plane of the sky, the Faraday rotation measures the sign and U data at both frequencies were corrected for the rotation and strength of the magnetic fields along the line-of-sight. The of the telescope reference frame of polarization with respect knowledge of the spectral index variations across radio contin- to the sky. At 8.35 GHz the distributions of Stokes parame- uum structures allows us to trace the history of relativistic elec- ters were combined into final Q and U maps using the same tron population transported with the gas and magnetic fields. technique as for total power data. At 4.85 GHz the Stokes U In the absence of an in-situ acceleration mechanism the rel- and Q data for each coverage from both horns were averaged, ativistic electrons lose their energy via synchrotron emission. then rotated to the RA/Dec frame and combined into final Q Since electrons with higher energies lose their energy more and U maps. A digital filtering process that removes spatial rapidly, the radio spectrum steepens with time (electron aging). frequencies corresponding to noisy structures smaller than the To obtain maps of Faraday rotation measures and spectral index telescope beamwidth was applied to final maps of all the Stokes we performed observations at two frequencies: 4.85 GHz and parameters at both frequencies. The Q and U maps were fi- 8.35 GHz. Additionally we used the Hi continuum at 1.4 GHz nally converted into maps of polarized intensity, accounted for obtained by Vollmer et al. (2004). The observations and the the “positive noise bias” in the way described by Wardle & data reduction are presented in Sect. 2. The results are shown Kronberg (1974). We also computed the distributions of appar- in Sect. 3 and discussed in Sect. 4. We give our conclusions in ent (i.e. uncorrected for Faraday rotation – small in our case) Sect. 5. polarization B-vectors, defined as arctan(U/Q) + 90◦. Instrumental polarization of the Effelsberg radio telescope forms specific “butterfly patterns” in Q and U maps at the level 2. Observations and data reduction of about 1% of an unpolarized signal (see Klein et al. 1982 Observations at 8.35 GHz and with a 1.1 GHz receiver band- for examples). They yield an almost axisymmetric structure in width were made using the single-horn receiver at the sec- polarized intensity and in orientation of polarization vectors, ondary focus of the 100-m Effelsberg radio telescope2.The constant in the reference frame of the telescope. Corrections galaxy was observed by making the maps (called coverages), for the azimuthal mounting makes the vectors of instrumental scanned alternatively in RA and Dec. A total of 22 cover- polarization rotate by the parallactic angle. In the case of obser- ages was obtained. Four data channels were recorded. The first vations at a wide range of parallactic angles (which is our case) two channels contain total power signals. The correlations of the superposition of spurious polarization patterns with vari- the left- and right-handed circular polarization signals (giv- ous angles reduces the influence of instrumental polarization ing Stokes Q and U) are recorded in the other two channels. by several times. With the unpolarized intensities discussed be- At 4.85 GHz we used the two-horn system in the secondary low any instrumental polarization is well below the noise. focus of the Effelsberg telescope with a receiver of 0.5 GHz The rms noise levels in total and polarized intensity at bandwidth. Four data channels (two Stokes I as well as Q 8.35 GHz are 0.25 mJy/b.a. and 0.045 mJy/b.a., respectively. / and U – see above) were recorded for each horn. We obtained At 4.85 GHz the corresponding noise levels are 0.4 mJy b.a. in / 10 azimuth-elevation coverages of NGC 4569. total intensity and 0.09 mJy b.a. in polarized intensity.
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